As I agree with not using cement as the counter balance I do not think some of the suggest replacement items (water tank or firewood) are ideal.
Say you have the generator on the back and fill a water tank or wood box up front. While after camping for a weekend and using the water or wood your counter balance is now gone and you still have a generator on the back.
If you are going to change it into usable storage, I would make sure it wasn't consumable items. Items you take with you each and every trip. Then your counter balance will be there on both ends of your trip. Also storing the same gear will give you a known and consistent weight of the counter balance.
I would also listen to a few others and hit the weight scales. I think a counter balance will most likely be needed for safety and maybe piece of mind but 600 lbs seems like a lot. After full water tank, 600 lb counter balance and a generator/fire wood on the back sounds like you would be close to your cargo capacity.
